This invention relates to vehicles and more particularly to a four-wheel steering system for vehicles.
Relatively large vehicles, employed for example as prime movers, tractors, utility vehicles, and material handlers, are often equipped with four-wheel steering to facilitate maneuvering in relatively confined areas. Such four-wheel systems often include a dual-acting hydraulic cylinder coupled to each wheel through a linkage wherein each wheel may be pivoted through a limited angle in either direction. In one type of four-wheel steering system, the two front wheel cylinders are connected in parallel to each other and the rear wheel cylinders are similarly connected. In addition, the parallel combination of front wheel cylinders are connected in series with the parallel combination of rear wheel cylinders. In such series-parallel systems, either the pair of front or rear wheel cylinders receives pressurized hydraulic fluid directly from the pressure source, while the other pair receives slave fluid expelled from the opposite ends of the directly connected cylinders. As a result of leakage, the directly connected cylinders receive more hydraulic fluid than the second pair of cylinders. When such four-wheel steering systems are in transition from steering in one direction to steering in an opposite direction, both the front and rear wheels will be moved through a forward or neutral orientation before they become oriented in the opposite direction. However, because the directly connected turning cylinders receive more hydraulic fluid than the slave-connected cylinders, one pair of wheels will reach a neutral or center orientation prior to the other set. This may result in a transitional situation wherein the front and rear wheels are temporarily oriented in opposite directions.